SSRv manuscript No.
(will be inserted by the editor)
arXiv:0801.0964v1 [astro-ph] 7 Jan 2008
Clusters of galaxies: beyond the thermal view
J.S. Kaastra · A.M. Bykov · S. Schindler ·
J.A.M. Bleeker · S. Borgani · A. Diaferio ·
K. Dolag · F. Durret · J. Nevalainen ·
T. Ohashi · F.B.S. Paerels · V. Petrosian ·
Y. Rephaeli · P. Richter · J. Schaye ·
N. Werner
Received: 7 September 2007; Accepted: 20 December 2007
J.S. Kaastra
SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, the Netherlands
Astronomical Institute, Utrecht University, P.O. Box 80000, 3508 TA Utrecht, The Netherlands
E-mail: j.kaastra@sron.nl
A.M. Bykov
A.F. Ioffe Institute of Physics and Technology, St. Petersburg, 194021, Russia
E-mail: byk@astro.ioffe.ru
S. Schindler
Institute for Astro- and Particle Physics, University of Innsbruck, Technikerstr. 25, 6020 Innsbruck, Austria
E-mail: Sabine.Schindler@uibk.ac.at
J.A.M. Bleeker
SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, the Netherlands
Astronomical Institute, Utrecht University, P.O. Box 80000, 3508 TA Utrecht, The Netherlands
E-mail: j.a.m.bleeker@sron.nl
S. Borgani
Department of Astronomy, University of Trieste, via Tiepolo 11, I-34143 Trieste, Italy
INAF – National Institute for Astrophysics, Trieste, Italy
INFN – National Institute for Nuclear Physics, Sezione di Trieste, Italy
E-mail: borgani@oats.inaf.it
A. Diaferio
Dipartimento di Fisica Generale “Amedeo Avogadro”, Università degli Studi di Torino, Via P.
Giuria 1, I-10125, Torino, Italy
Istituto Nazionale di Fisica Nucleare (INFN), Sezione di Torino, Via P. Giuria 1, I-10125,
Torino, Italy
E-mail: diaferio@ph.unito.it
K. Dolag
Max-Planck-Institut für Astrophysik, P.O. Box 1317, D-85741 Garching, Germany
E-mail: kdolag@mpa-garching.mpg.de
F. Durret
Institut d’Astrophysique de Paris, CNRS, UMR 7095, Université Pierre et Marie Curie, 98bis
Bd Arago, F-75014 Paris, France
E-mail: durret@iap.fr
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Abstract We present the work of an international team at the International Space
Science Institute (ISSI) in Bern that worked together to review the current observational and theoretical status of the non-virialised X-ray emission components in
clusters of galaxies. The subject is important for the study of large-scale hierarchical
structure formation and to shed light on the ”missing baryon” problem. The topics
of the team work include thermal emission and absorption from the warm-hot intergalactic medium, non-thermal X-ray emission in clusters of galaxies, physical processes
and chemical enrichment of this medium and clusters of galaxies, and the relationship
between all these processes. One of the main goals of the team is to write and discuss a
series of review papers on this subject. These reviews are intended as introductory text
and reference for scientists wishing to work actively in this field. The team consists of
sixteen experts in observations, theory and numerical simulations.
Keywords Galaxies: clusters: general · intergalactic medium · large-scale structure
of universe · X-rays: galaxy clusters
J. Nevalainen
Observatory, P.O. Box 14, 00014 University of Helsinki, Helsinki, Finland
E-mail: jnevalai@astro.helsinki.fi
T. Ohashi
Department of Physics, School of Science, Tokyo Metropolitan University, 1-1 Minami-Osawa,
Hachioji, Tokyo 192-0397, Japan
E-mail: ohashi@phys.metro-u.ac.jp
F.B.S. Paerels
Department of Astronomy and Columbia Astrophysics Laboratory, Columbia University, 550
West 120th Street, New York, NY 10027, USA
SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, the Netherlands
E-mail: frits@astro.columbia.edu
V. Petrosian
Department of Applied Physics, Stanford University, Stanford, CA, 94305
Kavli Institute of Particle Astrophysics and Cosmology, Stanford University, Stanford, CA,
94305
E-mail: vahep@stanford.edu
Y. Rephaeli
School of Physics & Astronomy, Tel Aviv University, Tel Aviv, 69978, Israel
Center for Astrophysics and Space Sciences, University of California, San Diego, La Jolla,
CA 92093-0424
E-mail: yoelr@wise.tau.ac.il
P. Richter
Institut für Physik, Universität Potsdam, Am Neuen Palais 10, D-14469 Potsdam, Germany
E-mail: prichter@astro.physik.uni-potsdam.de
J. Schaye
Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands
E-mail: schaye@strw.leidenuniv.nl
N. Werner
SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, the Netherlands
E-mail: n.werner@sron.nl
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1 Scientific rationale of the project
Clusters of galaxies are the largest gravitationally bound structures in the Universe.
Their baryonic composition is dominated by hot gas that is in quasi-hydrostatic equilibrium within the dark matter dominated gravitational potential well of the cluster.
The hot gas is visible through spatially extended thermal X-ray emission, and it has
been studied extensively both for assessing its physical properties and also as a tracer
of the large-scale structure of the Universe.
Clusters of galaxies are not isolated entities in the Universe: they are connected
through a filamentary cosmic web. Theoretical predictions indicate the way this web is
evolving. In the early Universe most of the gas in the web was relatively cool (∼ 104 K)
and visible through numerous absorption lines, designated as the so-called Lyα forest.
In the present Universe, however, about half of all the baryons are predicted to be
in a warm phase (105 − 107 K), the Warm-Hot Intergalactic Medium (WHIM), with
temperatures intermediate between the hot clusters and the cool absorbing gas causing
the Lyα forest.
The X-ray spectra of clusters are dominated by the thermal emission from the hot
gas, but in some cases there appears to be evidence for hard X-ray tails or soft X-ray
excesses. Hard X-ray tails are difficult to detect, and one of the topics for the team
is a discussion on the significance of this detection (yet contradictory) in existing and
future space experiments. Various models have been proposed to produce these hard
X-ray tails, and our team reviews these processes in the context of the observational
constraints in clusters.
While in some cases soft excesses in clusters can be explained as the low-energy extension of the non-thermal hard X-ray components mentioned above, there is evidence
that a part may also be due to thermal emission from the WHIM. The signal seen
near clusters then originates in the densest and hottest parts of the WHIM filaments,
where the accelerating force of the clusters is highest and heating is strongest. A strong
component of this emission is line radiation from highly ionised oxygen ions, and the
role of this line emission and its observational evidence will be reviewed.
WHIM filaments not only can be observed because of their continuum or line emission, but also through absorption lines if a sufficiently strong continuum background
source is present. The evidence for absorption in both UV and X-ray high-resolution
spectra is discussed. Future space missions will be well adapted to study these absorption lines in more detail.
In particular in absorption lines the lower density parts between clusters become
observable. In these low density regions of the WHIM not only collisional ionisation but
also photo-ionisation is an important process. In general, the physics of the WHIM is
challenging due to its complexity since there are many uncertain factors including the
heating and cooling processes, the chemical enrichment, the role of supernova-driven
bubbles or starburst winds, ram-pressure stripping, the role of shocks, magnetic fields,
etc. More detailed (and sophisticated) hydrodynamical simulations with state-of-theart spatial (and temporal) resolution are required in order to follow the impact of
some (if not all) of these important processes. In particular chemical enrichment is an
important process to consider as it leads to many observable predictions. We review
the various physical processes relevant for the WHIM, the methods that are used to
simulate this and the basic results from those models.
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2 Timeliness of this work
The first detections of non-virialised components in or between clusters of galaxies such
as thermal emission or absorption from the WHIM or the presence of hard X-ray tails
have now been made. There are several initiatives for new space missions to study the
physics of the WHIM, either in emission or absorption, from the USA, Japan, Italy and
The Netherlands, and new missions for studying hard X-ray tails are being designed in
France, Japan and the USA. Significant theoretical progress is being made in this field,
also thanks to the enormous leap in computing power for numerical models. In addition
to the considerable intrinsic interest in the astrophysics of clusters, these systems are
fundamental probes of the underlying cosmology and of the large-scale structure. This
is the time to put the expertise of observers and theoreticians in different fields together,
in order to review our current knowledge and make it available to the community in a
self-contained and comprehensive – yet concise – review volume.
3 Organisation of the team work
Our team consisted of sixteen members; two members unfortunately could not attend
both meetings; for one participant a replacement could be found. The people involved
in this international team were Xavier Barcons (Spain), Johan Bleeker (Netherlands,
co-organiser), Andrei Bykov (Russia, co-organiser), Stefano Borgani (Italy), Antonaldo Diaferio (Italy), Klaus Dolag (Germany), Florence Durret (France), Jelle Kaastra
(Netherlands, organiser), Jukka Nevalainen (Finland), Takaya Ohashi (Japan), Frits
Paerels (USA), Vahé Petrosian (USA), Yoel Rephaeli (Israël), Philipp Richter (Germany), Joop Schaye (Netherlands), Sabine Schindler (Austria, co-organiser), and Norbert Werner (Netherlands). In addition a few people outside the team helped in writing the review papers or even have taken up the lead in writing these papers: Serena
Bertone (UK), Chiara Ferrari (Austria), Federica Govoni (Italy), Dunja Fabjan (Italy),
Luca Tornatore (Italy), and Rob Wiersma (Netherlands).
Our team started with a one week meeting at the end of October 2006 at ISSI in
Bern. Each team member introduced a topic, and in the following discussion the relevant issues for the review were collected. After the discussions, subteams were formed
around the different topics, and the outlines of the different chapters as presented in
the present volume were defined. Each team member participated in several subteams
and papers, either as first author or co-author. In the time between the first and the
second meeting, several subteams were able to work out these drafts further. At the
second meeting, the progress was reviewed, comments were made on the drafts and the
remaining time was used to work further on the drafts.
After the second meeting, the draft papers were finished by the authors and sent to
two internal referees from the team who are non-experts in the field, in order to check
the comprehensibility of the papers and of course for the normal work of any referee.
All papers were then reviewed by an anonymous, external and expert referee. That
there was a lively discussion and interaction is proven by the more than 700 emails
that were exchanged between the authors and the editor between July and December
2007.
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4 About the title of the book
Our series of review papers both appears as a special volume of Space Science Reviews,
as well as in the form of a book published by Springer. One of the hardest items to
resolve was to find a catchy title for the book. At our last meeting in June 2007
we had a long and lively discussion about it without a clear outcome. Only shortly
before the deadline we converged to the title of the book (which was taken also as the
title of this chapter / paper). While discussing what is beyond the thermal view, we
spend of course also quite some time on thermal emission. It is obvious that without
a thorough understanding of the thermal aspects of the cluster gas, it is not well
possible to go beyond. But in this series of reviews we hope to demonstrate that
cluster physics is a rich subject, and that there is more beyond galaxies, dark matter,
and “just” hot gas with only one interesting parameter, its temperature (well, and its
emission measure), needed to estimate cluster masses. We go beyond this, and present
non-thermal emission, shocks, magnetic fields, chemical enrichment, and all the other
interesting processes shown on the front cover of the book. Also, we go beyond the
– somewhat diffuse – physical boundaries of clusters to the warm-hot intergalactic
medium with all its interesting astrophysical aspects. We hope that our book and the
individual papers help the readers also to go beyond what is written here, and we invite
them to explore the topics of this book further and to advance our understanding of
them.
5 Final remarks
The team members look back to an interesting and rewarding project. The time spent
at ISSI has been successful, both from a point of view of the project, the interactions
and the atmosphere surrounding the project. The help and assistance of ISSI and its
staff members has been invaluable for this work.
Acknowledgements The authors thank ISSI (Bern) for support of the team “Non-virialized
X-ray components in clusters of galaxies”. SRON is supported financially by NWO, the Netherlands Organization for Scientific Research. A.M.B. acknowledges the RBRF grant 06-02-16844,
a support from RAS Presidium Programs, and a support from NASA ATP (NNX07AG79G).
S.S. acknowledges financial support by the Austrian Science Foundation (FWF) through grants
P18523-N16 and P19300-N16, by the Tiroler Wissenschaftsfonds and through the UniInfrastrukturprogramm 2005/06 by the BMWF. A.D. gratefully acknowledges support from the
PRIN2006 grant “Costituenti fondamentali dell’Universo” of the Italian Ministry of University and Scientific Research and from the INFN grant PD51. F.D. acknowledges support from
CNES. J.N. acknowledges support from the Academy of Finland. F.P. acknowledges support
from the Dutch Organization for Scientific Research NWO. N.W. acknowledges support by the
Marie Curie EARA Early Stage Training visiting fellowship.
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Fig. 1 The team members in front of the ISSI building at the first meeting: Joop Schaye
(1), Klaus Dolag (2), Antonaldo Diaferio (3), Frits Paerels (4), Jukka Nevalainen (5), Vahé
Petrosian (6), Jelle Kaastra (7), Yoel Rephaeli (8), Andrei Bykov (9), Takaya Ohashi (10),
Norbert Werner (11), Florence Durret (12), Philipp Richter (13) and Sabine Schindler (14).